1. Field of the Invention
The present invention relates to a method and apparatus for the combustion of large particles.
2. The Prior Art
As early as in 1961 F. H. Reynst mentioned that it was known that acoustic vibrations have a beneficial effect on combustion. In this connection reference is made to Pulsating Combustion, pp 13-15, The Collected Works of F. H. Reynst, Pergamon Press, New York 1961. Although the vibrations may be only very weak, the relative motion of the gas with respect to the fuel particle which results is sufficient to remove the envelope of combustion products around this particle, resulting in an increase of the combustion rate. Reynst describes the application of this principle to a pulverized coal burner. A mixture of fuel and air is delivered by a fan to a precombustion chamber located between two conical passages flaring in the direction of flow. Volatile components of the fuel are combusted in the precombustion chamber, and the flame is directed into a flame tube. The pulsations of the flame in the precombustion chamber are propagated into the flame tube wherein the column of gas is set in resonance so as to move relatively with respect to the fuel particles, which speeds up the combustion as mentioned above.
Swedish patent specification No. 7701764-8 (publ. No. 412 635) describes a method of combusting atomized solid, liquid or gaseous fuels, which is based on the principle mentioned by Reynst. However, according to this patent specification the vibrations are not generated by the burner flame. Sound energy is supplied to the combustion flame by external means such as a sound emitter, the frequency of the sound ranging from infrasound frequencies to ultrasound frequencies. However, the method of the Swedish patent specification No. 7701764-8 apparently has not yet been utilized practically to any significant extent, which may indicate that it has not been possible so far to develop the method for industrial application.
Similar methods are described in Swiss Pat. No. 281,373 and German Pat. No. 472,812. According to the Swiss patent, vibration is imparted to at least part of the combustion chamber and the flue gases, and according to the German patent, a dispersion of particulate fuel and combustion air as well as secondary combustion air is brought to oscillation.
The USSR Author's Certificate No. 228,216 (V. S. Severyanin) describes a pulsating combustion in a bed whereby the hot grid of the Rijke tube is replaced by a layer of solid fuel in which free oscillation will develop. The effect obtained is, however, relatively low, because only self-generated oscillation is utilized.
U.S. Pat. No. 1,173,708 describes a method for burning fuel wherein the particles of a fuel bed laying on a grate are agitated by pulsating combustion air supplied from below through the grate. The particles of fuel are suspended and floated by the air and are permitted to settle in the time intervals between the pulsations.